1. Field of the Invention
The present invention relates to a filter circuit for an integrated circuit, and more particularly to an active filter for operation at a low supply voltage.
2. Description of the Related Art
A filter circuit is intended for removing unnecessary components and accepting necessary components from various frequency components of a signal. There are a large variety of filter circuits. Roughly classified, the filter circuits can be divided into passive filters, active filters, and digital filters.
An active filter comprises passive devices, such as a capacitor (C) and a resistor (R), and active devices, such as a transistor and an operational amplifier. Particularly, in the voice frequency band, CR feedback type filters including C and R and active devices are used very widely. Meanwhile, in this frequency band, the time constant of a CR coupling circuit is long, the capacitor is required to have a large capacity, and above all, the resistor is required to have high accuracy. Therefore, when a filter circuit is formed as a semiconductor integrated circuit, it is difficult to mount the C and R in the same single semiconductor substrate.
This inventor knows an integrated circuit applicable to solve the above problem, in which integrated circuit an active filter is configured by using a voltage-input current-output type amplifier, namely a transconductance amplifier, with the aim of reducing the required number of component parts.
As an example of such a circuit, FIG. 1 shows a primary low-pass filter. In FIG. 1, a transconductance amplifier 40 comprises transistors Q41 to Q49, resistors RE and RE, and constant-current circuits CS41 to CS43. Reference numerals 41 and 43 denote an input terminal and an output terminal of the low-pass filter. A capacitor 42 is connected between an output line 45 of the transconductance amplifier 40 and a ground (GND) line 46, and the output line 45 is connected to an input terminal of a buffer circuit 44 with an amplification factor of 1.0. Output of the buffer circuit 44 is connected to the output terminal 43 and the base of a transistor Q42, which is a component part of the transconductance amplifier 40. The transistors Q47 to Q49 of the transconductance amplifier 40 constitute a Wilson current mirror circuit, the mirror factor of which is a value close to 1.
The minimum operating voltage VCC.sub.MIN of an active filter of this kind is determined by a voltage across a constant-current source CS42, the collector-emitter voltage of the transistor Q46, the base-emitter voltage of the transistor Q49, and the base-emitter voltage of the transistor Q48. Therefore, when the constant-current source CS42 is formed by one transistor and the collector-emitter voltage V.sub.CE of the transistor is 0.4 V with the base-emitter voltage V.sub.BE at 0.6 V, the minimum operating voltage VCC.sub.MIN is given as shown below. EQU VCC.sub.MIN =V.sub.CE (CS42)+V.sub.CE (Q46)+V.sub.BE (Q49)+V.sub.BE (Q48)=0.4+0.4+0.6+0.6=2.0 V
Meanwhile, when a filter such as this is mounted in a battery-driven integrated circuit, the circuit should preferably configured so as to operate at a supply voltage of about 1.8 V. For this reason, a filter circuit as shown in FIG. 1 not suitable for this application.
In a differential amplifier formed by the transistors Q41 and Q42, a voltage drop occurs at the resistors RE and RE connected to the emitters of those transistors, so that the minimum operating voltage is decreased by the value of the voltage drop. Also in this respect, the filter circuit shown in FIG. 1 is not suited for operation at a low supply voltage.